When attempting to activate your core, the BRACE is far more effective than the HOLLOW. Bracing is the act of stiffening all three levels of the core simultaneously to create a 360 degree stabilization of the torso. Hollowing is an attempt to isolate a single muscle of the core (transverse abdominus), giving the exerciser a flexion response and very little overall stability. Think of a circus tent with a single pole in the middle (your spine). The guy wires that are attached to the pole at the corners and then to the ground give a buttressing effect that leaves the pole (your spine) stable. This is what happens when you brace. The muscles of the core work as a unit, giving equal amounts of support from all angles, resulting in superior stability.
Matt Pirtle MA CSCS